This invention relates to a method for manufacturing an optical probe for the invasive measurement of blood parameters, said optical probe comprising at least one sensor sensitive to a blood gas parameter such as pO.sub.2 or pCO.sub.2 and at least one additional sensor sensitive to the pH value of the blood, said sensors having selective membranes, and said optical probe further comprising a sheath at least partially covering said sensors and fastened on said sensor by a glue.
Probes for the invasive measurement of blood parameters consist of at least one sensor comprising an optical fiber, said fiber ending up with a gel zone containing a dye. The optical density or another optical parameter of said dye varies with the blood parameter (such as pH) to be measured. On the other side of the dye-containing gel, a reflector is positioned. The end of the fiber, the gel and the reflector are surrounded by a semi-permeable envelope (for example, a hydrogen ion permeable envelope in the case of a pH sensor) to keep the gel in place.
Light from this optical fiber passes the dye-containing gel, is reflected by said reflector, passes the gel again and is transmitted through the optical fiber to an appropriate detector which measures light attenuation or changes in other optical parameters caused by the dye. This attenuation or change is a function of the blood parameter to be measured and the relation between attenuation, absorbance or the change of another optical parameter and the blood parameter is well-known.
Such a probe can be introduced into a patient's artery to measure--depending on the dye--various blood parameters such as pH, pO.sub.2 or pCO.sub.2.
For a more detailed description of fiber optic pH measurement, reference is made to "A Miniature Fiber Optic pH Sensor for Physiological Use", Journal of Biomechanical Engineering, May 1980, pg. 141 ff.
It is a major goal of probe development to combine more than one sensor in a single probe. The reason is that a patient should not be overstrained with various probes introduced in his arteries. Such a combination or multiple sensor probe may, for example, contain a pH sensor, a pO.sub.2 sensor, a pCO.sub.2 sensor and/or a stabilizing element such as a wire.
In the case of a multiple sensor probe, the sensors have to be coupled mechanically. This can be achieved by use of a sheath covering the front end of the probe and being appropriately perforated to allow the ions (in the case of a pH sensor) or the gas molecules (in the case of pO.sub.2 or a pCO.sub.2 sensor) to reach the permeable envelope of the sensor, pass it and diffuse into the dye-containing gel. The sheath has to be secured by a glue or adhesive.
A serious disadvantage when manufacturing such probes is that--when the glue is applied to the sensor--the glue tends to move or creep along the same, thereby covering the diffusion zones of the selective membranes, i.e. the permeable envelopes in the region of the dye-containing gel. The result is that the ions or gas molecules cannot or can hardly reach the selective membrane. Such a probe is either insensitive or has a very long time constant in the range of half an hour or more which makes it unusable.